Abstract
The Atkinson cycle, where expansion ratio is higher than the compression ratio, is one of the methods used to improve thermal efficiency of engines. Miller improved the Atkinson cycle by controlling the intake- or exhaust-valve closing timing, a technique which is called the Miller cycle. The Otto–Miller cycle can improve thermal efficiency and reduce NOx emission by reducing compression work; however, it must compensate for the compression pressure and maintain the intake air mass through an effective compression ratio or turbocharge. Hence, we performed thermodynamic cycle analysis with changes in the intake-valve closing timing for the Otto–Miller cycle and evaluated the engine performance and Miller timing through the resulting problems and solutions. When only the compression ratio was compensated, the theoretical thermal efficiency of the Otto–Miller cycle improved by approximately 18.8% compared to that of the Otto cycle. In terms of thermal efficiency, it is more advantageous to compensate only the compression ratio; however, when considering the output of the engine, it is advantageous to also compensate the boost pressure to maintain the intake air mass flow rate.
Highlights
In the mid-1940s, Miller proposed the concept of making the expansion ratio higher than the compression ratio through a the decrease in the effective compression ratio due to the change in valve closing time, without significant structural changes in the existing engine
By substituting the temperature and mass change in each state described above and considering it as an intake and exhaust process involving a quasi-static change ( Pi = Pe ), the theoretical thermal efficiency is expressed as follows, when the Otto cycle is converted into the Miller cycle by changing the intake-valve closing timing
When compensating only the compression ratio (ε∗c ), pressure ratio (α∗ ) and the Mean Effective Pressure (MEP) tends to decrease owing to the continuous reduction in the intake air mass, the thermal efficiency is improved up to 430 ◦ crank angle (CA)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. In the mid-1940s, Miller proposed the concept of making the expansion ratio higher than the compression ratio through a the decrease in the effective compression ratio due to the change in valve closing time, without significant structural changes in the existing engine This technique highlights the usefulness of increasing the thermal efficiency of engines by applying the Atkinson cycle. This cycle is an over-expanded cycle, i.e., one with a higher expansion ratio than compression ratio It has recently been proposed as a means of reducing hazardous emissions while maintaining engine efficiency by lowering the engine compression rates and maximizing the gas temperature and pressure in cylinders. Processes 2021, 9, 1051 sion ratio than the compression ratio, with improved thermal efficiency compared to the operating conditions of a conventional internal combustion engine [34] This difference in expansion ratio can be achieved through a compression stroke that includes a late or early closing of the intake valve. EIVC, which is considered more effective in reducing NOx emissions [43,44,45], was applied in this cycle analysis
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